config(8) should check if a scheduler is selected

[Terry Lambert]

Bruce Evans wrote:
> It is the only mandatory option (sic).  Kernels with no options (although
> they might not be useful) can be built except for this bug.  Example of
> a minimal config file (before misconfiguration of the configuration of
> scheduling).

Make the scheduler code stop moving for a good week, and I am
willing to fix this with linker sets and an opaque indirection
interface.  It won't fix the primary problem of coexistance, at
least up front, but it could allow you to select your scheduler
in loader.conf (-current should remain buildable and runnable in
that same week, of course).

-- Terry